Previously known optical time slot interchangers tended to fall into two widely separated categories. At one extreme is the optical fiber intensive 1.times.n splitter/n.times.1 combiner OTSI, known as a loopless architecture from the article "Architectures with Improved Signal-to-Noise Ratio in Photonic Systems with Fiber-Loop Delay Lines" by R. A. Thompson, published August 1988 in IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS. It is called loopless because there is no looping back or re-circulation used in obtaining the delays. This type of OTSI has n individual lengths of optical fiber, the shortest being a one time slot delay and the longest being an n slot delay. The length of each of the n fibers being one slot time longer than the previously shorter delay, for example 1, 2, 3 . . . n-1, n. The loopless architecture provides an OTSI for a frame of N time slots. For n=16, the largest possible frame would have 8 time slots for a frame integrity maintaining OTSI and the total of all fiber delays would be 136 time slot periods. Such an OTSI is shown as system 10 in FIG. 1.
At the other extreme is the single stage Thompson Re-entrant memory loop design which was able to achieve any of N possible delays for N time slots by implementing n separate delays of one time slot each. Delays greater than one time slot are provided by switching circuits recirculating the signals of the time slot within its respective delay fiber. N delays are required under the worst case condition of delaying the first slot of data for N-1 slot times and inserting it at the end of the frame. Such an OTSI is shown as system 20 in FIG. 2 and is also described in the above referenced article.
The Thompson Reentrant type of OTSI 20, however, has significant attenuation and SNR limitations because the signals of some time slots are looped through the same delay fibers and their associated switching elements up to N times. During each circulation, the optical signals of the particular time slot being delayed are attenuated by the attenuation of the delay fiber and are subjected to an insertion loss of the switching element. These attenuation and SNR limitations cause this OTSI architecture to be considered by many to be impractical, even though it uses the smallest amount of optical fiber of any of the OTSI architectures.
Thus, there is a need in the art for an N time slot OTSI that has few fiber delays and yet does not have severe attenuation and switching losses. An N time slot OTSI supports all delays between 0 time slots and N-1 time slots and supports all interchange permutations of time slots within a frame, including the worst case of all N time slots being delayed by N-1 time slot positions. This means that the total fiber length must be capable of storing N-1 time slots of information simultaneously, in addition to providing a maximum delay of N-1 time slots. Additionally, the total fiber delay must provide combinations that deliver every possible delay between 0 and N-1 time slots without the signals of any two time slots requiring the same delay fiber simultaneously.
It is an object of the present invention to provide an N time slot OTSI that has the desired characteristics listed above and is constructed with the minimum total length of optical fiber.
It is another object of the present invention to provide an N time slot OTSI that is constructed with the minimum length of fiber without the signals of any slot re-circulating through the same delay fiber and associated switching elements twice.
It is another object of the invention to provide an N time slot OTSI that is capable of supporting all permutations of time slots in a frame, the worst case being that all N time slots are to be delayed by N-1 time slot position, which means that the total fiber length must be capable of storing N-1 time slots of information simultaneously in addition to providing a maximum delay of N-1 time slots.
It is another object of the present invention to provide an OTSI in which the total length of fiber is partitioned in a manner to achieve combinations that deliver every possible delay between 0 and N-1 time slots without the signals of any time slot re-circulating through the same delay fiber and associated switching elements twice.